Physiological waveform processors are known to the prior art for such purposes as detecting the occurrence of a particular waveform characteristic. For example, such devices have been used to detect the occurrence of an R-wave or a QRS complex within an ECG.
Typical prior art devices of the type described recognize that the slope or slew rate of the waveform characteristic of interest often differs from that of the remainder of the waveform. When the characteristic of interest has a greater slew rate than other waveform characteristics, a mere comparison of the derivative of the waveform signal against a reference value can reliably detect the occurrence of the characteristic of interest. In an environment where noise or other extraneous signals are likely to be present, a second criterion may be employed to reduce the probability of a false indication of the waveform characteristic of interest.
A prior art device which employs the slope or slew rate of a waveform to detect the occurrence of a particular waveform characteristic is shown in U.S. Pat. No. 3,878,833, issued Apr. 22, 1975, for Physiological Waveform Detector. The referenced patent includes an embodiment which detects the occurrence of the QRS complex in the ECG waveform essentially by comparing the derivative of the waveform signal with a reference signal and determining whether the derivative signal exceeds the reference signal for a predetermined period of time. Thus, two criteria are employed to detect the QRS complex: namely, magnitude and duration. The magnitude criterion distinguishes between the QRS complex and other portions of the ECG as well as low frequency artifacts and noise. The duration criterion distinguishes between the QRS complex and muscle spikes, pacemaker pulses and other high frequency artifacts.
Prior art devices of the type disclosed in the referenced patent can detect a QRS complex in an ECG and other waveform characteristics within other waveforms. The process of detection by comparison with a reference value, however, merely gives an indication of the occurrence of the characteristic of interest without any indication as to the parameters of that characteristic. Further, the criteria employed by this prior art device may not be the most reliable or desirable to establish the occurrence of the characteristic. For example, in determining whether or not a cardiac pacing lead is properly placed, peak-to-peak amplitude and slew rate may be far more valuable than the magnitude and duration of a waveform characteristic.
A signal processor which establishes peak-to-peak amplitude between a waveform signal peak and the next successive peak as well as the slew rate of the waveform signal between successive waveform signal peaks is disclosed in United States Patent Application Ser. No. 180,710, filed Aug. 25, 1980, for Physiological Waveform Processor which is co-owned with the present invention. This system determines the maximum peak-to-peak amplitude of the waveform signal and its associated slew rate. The occurrence of a particular waveform characteristic is detected by comparing the maximum peak-to-peak amplitude and its associated slew rate against reference values. If the reference values are satisfied, the system may then indicate that the waveform characteristic has, in fact, occurred. For example, comparison of the maximum peak-to-peak amplitude, and its associated slew rate, against reference values may be employed to detect the occurrence of a QRS complex in an ECG waveform, as in a sense amplifier or cardiac pacemaker, or to check the lead placement for a cardiac pacemaker. However, this system requires that that portion of the waveform being processed that has the maximum peak-to-peak amplitude also has the greatest merit or probability that it is the characteristic of interest. In fact, a noise spike may have a higher peak-to-peak amplitude than other portions of the waveform being processed. Such a noise spike would not likely be detected as a QRS complex as a result of filtering and the slew rate comparison. However, if detected, it could prevent a detection of the QRS complex as a result of its higher peak-to-peak amplitude. Thus, this latter mentioned system can reliably give an indication that it has detected as QRS complex but may not always reliably detect that complex. Further, it is limited to a detection of those complexes having the highest peak-to-peak amplitude in the waveform.